Technical Abstract:
Citrus yellow mosaic virus (CYMV) causes yellow mosaic disease of citrus, the most widely grown fruit crop in the world, in India. CYMV belongs to the new Badnavirus genus because it has a nonenveloped bacilliform particle and double-stranded DNA genome. We have cloned an isolate of CYMV and determined its genome to be 7559 bp in length. Its genome contains 6 putative open reading frames capable of encoding proteins involved in viral movement, capsid synthesis, replication and transmission by insects. Phylogenetically, CYMV is most closely related to cacao swollen shoot virus. To determine whether the cloned CYMV genome is infectious in sweet orange, we constructed pBICYMV, a clone containing 1.4 copies of the CYMV genome in the binary vector pBI101.2, and introduced pBICYMV into sweet orange by Agrobacterium-mediated inoculation. We detected CYMV-specific products by PCR from, and observed bacilliform virus particles by immunosorbent electron microscopy in, extract of leaves distal to the inoculation site of all inoculated plants. We also observed typical yellow mosaic symptoms in some of the inoculated citrus plants, demonstrating our cloned CYMV genome is infectious. Our successful delivery of an infectious CYMV clone to sweet orange will facilitate further molecular studies of this virus by making possible the introduction of mutated viral genes into host plants to test the effect of mutations in planta. This is an important step toward a better understanding of this exotic plant pathogen in order to control and prevent the disease it causes in India from becoming established in other parts of the world.